We know a lot about the history of life on Earth, but how it began is still one of our greatest scientific mysteries. One hypothesis is that life actually originated on another planet, and many scientists today take the idea quite seriously. Though it sounds like the plot from recent scifi movie Prometheus, it's an old idea that even the celebrated nineteenth century physicist Lord Kelvin and Nobel winning geneticist Francis Crick have advocated. That's right — the evolution of life might have its beginnings on another planet.

Over 120 years ago, Kelvin shocked the British scientific community in a speech about what he called "panspermia," where he suggested that life might have come from planets smashing into each other and sending bits of life hurtling through space. He and a few colleagues had hit upon this notion after observing the massive 1880 eruption of a volcano on Krakatoa. To be more precise, they observed the aftermath of the volcano, which completely sterilized the island. No life was left at all. But then, within months, seedlings began to sprout and life took hold again.

Where had that life come from? To naturalists of the nineteenth century, it was obvious that it had drifted there from nearby islands. Seeds and insects blown on the wind, or floating on the tides, had begun the process of re-greening the stricken landscape. This got Kelvin thinking about the origin of life on Earth. Couldn't the same thing happen to barren planets drifting in space? Perhaps life had drifted to Earth on the stellar winds.

Aliens Seeded the Planet with Life

Today, we know that most life wouldn't survive the trip through space. It would be bombarded by radiation and subjected to hard vacuum. But Francis Crick, who was one of the first biologists to identify the structure of DNA, suggested a way around this problem. In a 1972 paper he co-authored with biologist Leslie Orgel called "Directed Panspermia," Crick suggested that perhaps extraterrestrials had seeded the Earth with microbes sent in specialized spaceships that would protect the microbes. This is an idea we see a lot in science fiction, including Prometheus. Still, Crick and Orgel didn't imagine aliens dribbling DNA into our water supply — they suggested it might have been sent out in automated probes, perhaps with a kind of "missionary zeal."

The problem, which Crick and Orgel discuss in the paper, is that it's incredibly hard to prove this hypothesis, or even to gather evidence one way or the other. That's why most scientists who study panspermia don't have much to say about the directed panspermia scenario. "It's not completely ridiculous," Purdue geophysicist Jay Melosh told io9. "It's fun to speculate about, but it's not the subject of really respectable scientific research because there's no evidence."

That said, Melosh and many other scientists do think panspermia might be part of the solution to the mystery of how life began.

We Come from Mars (or Europa)

Directed panspermia is simply the most unlikely version of a story that is actually quite plausible. Take out the aliens and the spaceships, and you still have many possible ways that microbes from other worlds might have made it to Earth. And if those microbes came from nearby, the panspermia scenario becomes even more plausible.

Cal Tech geologist Joe Kirschvink has suggested that Mars is a likely origin for life in the solar system because it would have been habitable long before Earth was. 4 billion years ago, when Earth was still a roiling cauldron of methane and magma, Mars was a stable, cool planet covered in vast oceans. It would have been the perfect place for microbial life to take hold. But how did that life make it all the way from the seas of Mars to the seas of Earth? Most likely, meteorites crashing into Mars would send fragments of the planet's surface back into space — packed with millions of microbes. In fact, around the time that Mars might have been developing life, the solar system was undergoing what astronomers call the "late heavy bombardment," a time of countless intense meteorite strikes.

Purdue geologist Melosh, who has spent most of his career studying meteorite impacts, has actually done experiments where he and a team recreated what might have happened when meteorites slammed into Mars billions of years ago, sending ejecta out of the atmosphere and eventually all the way to Earth. This process is sometimes called "ballistic panspermia," or "lithopanspermia," because it depends on rocks being ejected into space. To recreate one part of this process in their experiments, Melosh and his team shot a bacteria-covered rock with an aluminum projectile moving at 5.4 km per second, and the shattered chunks flew over a kilometer. The bacteria survived the trauma of what Melosh and his team called "extremes of compressional shock, heating, and acceleration."

After several of these tests, Melosh and his colleagues were certain that microbes could survive one of the most destructive parts of the ballistic panspermia journey:

A lot [of the microbes] would die, but a lot would survive in a dormant state. Their journey would take possibly millions of years. But it's as if atmospheres are almost designed for this transfer of life. The meteorite comes from Mars, full of microbes protected from radiation by the rock. It enters Earth's atmosphere, and as it comes in at high speed the outside melts because of friction and gets hot, but the inside is protected just like a spacecraft capsule. The microbes inside are protected. Then the aerodynamic forces in the lower atmosphere fracture the meteorite, exposing the interior.

The rock fragments rain over the land, and the surviving microbes can take hold.

Most scientists who subscribe to this idea suggest that Mars is the likely source for a ballistic panspermia event, though Melosh isn't ruling out Jupiter's moon Europa either. Astronomers believe Europa harbors vast oceans beneath a thick layer of ice, and it's very possible that a meteorite could have crashed there, sending microbe-laced chunks of rocky ice into the inner solar system. Still other scientists suggest that life could even jump from one star system to another, and a recent paper on the topic explores how this could happen in star clusters. Still, it's not likely that Earth was seeded from beyond the system — unless aliens were behind it.

A Valid Scientific Hypothesis

A big question is why scientists are entertaining this idea at all. Doesn't it seem outlandish? Perhaps, but then again so is the sudden appearance of life on Earth over 3.5 billion years ago. How did we go from lifeless puddles of chemicals to strings of self-reproducing DNA on a planet that was at the time so inhospitable?

Panspermia doesn't answer this question — we still aren't sure how the life switch got flipped — but it could help explain the conditions where that life evolved.

1. The geological evidence for the earliest life on Earth is very early, soon after the end of the late bombardment. There is good evidence for life on Earth at 3.5 billion years ago, indirect evidence at 3.8 billion. The end of the late heavy bombardment is 3.8 billion years ago.

2. The genetic evidence indicates that the last universal common ancestor (LUCA) of life could have been roughly 3.5 billion years ago (but with large uncertainties) and that LUCA was a fairly sophisticated life form in terms of metabolic and genetic capabilities.

1 and 2 together give the impression that life appeared on Earth soon after the formation of suitable environments and it appears to have come in being remarkably developed - like Athena born fully formed from the head of Zeus.

3. Rocks from Mars have traveled to Earth and the internal temperatures experienced in these rocks during this trip would not have sterilized the interiors. Thus in principle life can be carried from Mars to Earth.

4. Mars did not suffer the large Moon-forming impact that would have been detrimental to the early development of life on Earth.

3 and 4 have lead to the suggestion that Mars would have been a better place for life to start in the early Solar System and it could have then been carried to Earth via meteorites.

5. Organic molecules are widespread in comets, asteroids, and the interstellar medium.

6. Comets could have supported subsurface liquid water environments soon after their formation due to internal heating by decay of radioactive aluminum.

7. As comets move past the Earth they shed dust which settles into Earth's atmosphere.

5, 6 and 7 have lead to the suggestion that life could have started in the interstellar medium or in small bodies such as comets and then been carried to the Earth by comet dust.

So, yes panspermia is a valid scientific hypotheses and warrants further investigation.

We Are Not Alone

And as we engage in that investigation, maybe we'll discover more than we bargained for. After all, if we owe our existence to life on other planets in our solar system, that makes a strong case for life outside it too. SETI astronomer Jill Tarter told io9 via email:

I think that intelligent life here on Earth is a proof of concept that it could exist elsewhere, but we will not know unless we search systematically and exhaustively enough to accumulate sufficient information to justify significant null result. Remember the last sentence of the 1959 Cocconi and Morrison paper [published in Nature]: "The probability of success is difficult to estimate, but if we never search, the chance of success is zero."

The search for life's origins on Earth could ultimately lead to the discovery of extraterrestrial life, too.